1. Field of the Invention
The present invention relates to high frequency lamps, and lighting devices using them including lighting circuits for driving the lamps.
2. Description of Related Art
High frequency discharge lamps, such as the fluorescent lamp are well known and popular. They are well known for their relatively cool temperature during operation, but they are not as efficient as desired and suffer from flickering.
Luminescence efficiency improves with the use of high frequency, the flickering of the brightness reduces the discharge lamp, and it is the lighting to the instant. Moreover, the lighting unit has become small and lightweight. However, early high frequency lighting circuits have a low power factor, and a problem of higher harmonic distortion of the lower frequency AC power supply. In order to solve this problem, there has been provided an inverter type power supply as described in Japanese Provisional Publication No. 4-193066. The arrangement described therein has the following circuit arrangement. A rectifier rectifies the AC power supply. A first capacitor smoothes the output of the rectifier through an inductor. An in-series circuit includes first and second switching devices connected in parallel with the first capacitor. A load circuit is connected through a second capacitor between the connecting point of the output end of the rectifier, and the inductor, and the connecting point of the first and second switching devices. The inductor shares the difference of the output voltage of the rectifier, and the voltage of the first smoothing capacitor. When the input voltage from the AC power supply is lower than the voltage of the first capacitor, input current flows. Higher harmonic distortion of input current is reduced, input current wave type is used as input voltage and the similar type, and the input power factor is improved. Inrush current near the peak value of power supply voltage is lowered by the inductor.
A switching power supply arrangement is shown in Japanese Provisional Publication No. 8-149816. The switching power supply has a bridge shaping circuit which rectifies the commercial power supply. The smoothing capacitor smoothes the output of the bridge shaping circuit. The switching element is intermittent in the voltage supplied from the smoothing capacitor. The intermittent switching output is supplied to the primary winding of the transformer. The switching power supply obtains the DC output from the secondary side of the insulating transformer. A low pass filter in the normal mode includes a filter choke coil and a filter capacitor is provided in the rectification current path of the bridge shaping circuit. The resonance capacitor is connected with the primary winding and forms the in-series resonance circuit. The total of the electrostatic capacity corresponding to the resonance capacitor and the electrostatic capacity forms the same first capacitor and two second capacitors. The first capacitor is connected to the output line of the bridge shaping circuit. The two capacitors are connected to the positive/negative input terminal of each bridge shaping circuit, respectively. The switching output from the primary winding is supplied to the bridge shaping circuit. In this switching power supply circuit, the total of the inductance of the primary winding of the insulating transformer and the electrostatic capacity of two second capacitors provides an in-series resonance which determines the oscillation frequency of the switching power supply circuit. The in-series resonance capacitor comprises one first capacitor and two second capacitors, and it connects with the bridge shaping circuit side. The switching output may be supplied to the shaping circuit side, and the power factor may be improved.
Another power supply arrangement, different from the first one described in the '066 publication is described in Japanese Provisional Publication No. 10-271848, published after the filing date of this application on which priority is claimed. A full wave rectifier rectifies the AC power supplied. A first smoothing is connected between the DC output ends of the full wave rectifier. First and second switching devices are connected to the first capacitor in parallel and ON/OFF is carried out at a frequency higher than that of the AC power supply. A pair of rectifiers are connected to the first and second switching devices in parallel, respectively. The in-series circuit of the second capacitor is connected to both ends of the AC power supply, and the third capacitor. The load circuit is connected between the connecting point of the first switching device and the second switching device, and the connecting point of the second capacitor and the third capacitor. A fourth capacitor is connected in parallel with at least one rectification device which comprises the full wave rectifier.
In the '066 publication, the high frequency concentrates and flows only to a pair of near diodes to which the load circuit is connected among the rectifiers by the side of the input. When there is much load current, the diodes experience a large temperature rise. Moreover, since load current flows to the inductor, in proportion to the current capacity of load, the inductor must be large. It becomes impossible to ignore power loss by the inductor. While power conversion efficiency falls after all, it is disadvantageous also from the small and lightweight viewpoint of equipment.
In the arrangement shown in the '816 publication, if the capacitor which resonates with the inductance comprises first and second capacitors when it is going to divert this to the lighting circuit temporarily, in order to start the discharge lamp, it is difficult to obtain necessary filament preheating and necessary secondary release voltages.
Furthermore, in the prior application, the fourth capacitor is connected in parallel with the rectification device which comprises the full wave rectifier. The voltage which the fourth capacitor applies becomes the value twice the route of the peak value of AC voltage. Since the capacitor with high voltage-proof needs to be used, it becomes expensive.